The present invention relates to an optical communication system such as an optical CATV, optical ITV, or mobile communication system, which connects one main station and a plurality of sub-stations via an optical fiber and, more particularly, to an optical communication system which uses an optical wavelength multiplex technique in upstream signals from a sub-station to a main station, and a method of controlling the optical communication system.
Mobile communications represented by cellular phones, PHSs (personal handyphone system), and the like use radio, and have prevailed in recent years. However, since mobile communications use radio, they are often disrupted in a given area such as an underground mall, tunnel, dead zone of a base-station antenna in the shade of a building, and the like, where a radio wave is hard to reach.
In order to cover such uncommunicatable areas, sub-stations that are small-output stations are placed at various locations corresponding to the dead zones of a radio wave, thus providing communicatable areas to assure convenience for the users.
In this manner, a large number of sub-stations (base stations) that support the mobile communication system are placed in, e.g., an underground mall, tunnel, shade of a building, and the like where a radio wave is hard to reach. It is most preferable in terms of cost to feed a radio signal to such areas via optical fibers, and to use simple sub-stations (base stations) each having an antenna port module alone, as described in the article xe2x80x9cThe New Generation of Wireless Communications Based on Fiber-Radio Technologiesxe2x80x9d (IEICE Transaction on Communications vol. E76-B, no. 9, September 1993). Sub-stations connected via optical fibers are receiving lot of attention in an optical ITV (Industrial Televisions), CATV, and the like based on cable transmission, since a large number of sub-stations can be placed at various locations.
An optical network technique that accommodates a plurality of distributed sub-stations in a main station still suffers problems. One serious problem is beat noise produced upon interference of light sources of a plurality of sub-stations. The beat noise will be explained below.
Assume that optical signal A originating from a given sub-station is located at a wavelength position separated xcex94xcex from optical signal B originating from another sub-station, as shown in FIG. 1A. When these optical signals are simultaneously received by a single receiver, beat noise due to optical signals A and B is produced at a higher frequency position, xcex94xcex, than the information signal band, as shown in FIG. 1B.
At this time, if the wavelengths of optical signals A and B are sufficiently separated from each other, i.e., if xcex94xcex is small, beat noise falls within the information signal band, i.e., beat noise is produced within the information signal band, thus deteriorating reception sensitivity. In the worst case, these signals cannot be received at all.
Hence, in order to suppress beat noise, a wavelength multiplex technique that assures a given wavelength spacing between sub-stations is required.
Note that in the required wavelength multiplex technique, wavelengths need not be assigned at high density and their spacing need only be controlled to prevent beat noise from falling within the signal band, unlike in a technique used for a trunk system for long-distance transmission.
However, since the wavelengths may change due to changes in atmospheric temperature, and beat noise may influence the information signal, a means for detecting beat noise and means for controlling the wavelengths are required. Furthermore, which of a plurality of sub-stations has caused beat noise must be specified.
As the above-mentioned wavelength multiplex transmission system, for example, Jpn. Pat. Appln. KOKAI Publication No. 9-83434 proposed a system in which a main station comprises a beat noise detector. This invention has an arrangement shown in FIG. 2, and the main station has a beat detector. More specifically, referring to FIG. 2, reference numeral 1 denotes a main station; 2-1, 2-2, 2-n, . . . , sub-stations; 3a, an optical fiber for transmitting an upstream optical signal; and 3b, an optical fiber for transmitting a downstream optical signal. The optical fiber 3a for transmitting an upstream optical signal forms a transmission path from the sub-stations 2-1, 2-2, 2-n, . . . toward the main station 1, and the optical fiber 3b for transmitting a downstream optical signal forms a transmission path from the main station 1 toward the 2-1, 2-2, 2-n, . . . .
Reference numeral 4 denotes an E/O (electro-optical) converter for a sub-station; 6, a branch optical fiber; 7, a photocoupler; 8, a sub-station controller; 9, a sub-station modulator; 10, an O/E (opto-electric) converter for a sub-station; and 11, a sub-station demodulator. Each of the sub-stations 2-1, 2-2, 2-n, . . . incorporates these devices.
Each of the sub-stations 2-1, 2-2, 2-n, . . . is connected to the optical fiber 3a for transmitting an upstream optical signal by its E/O converter 4 via the branch optical fiber 6. A plurality of photocouplers 7 are connected to the optical fiber 3a. When the distal ends of the branch optical fibers 6 are connected to these photocouplers 7, they are optically connected to each other.
Also, another plurality of photocouplers 7 are connected to the optical fiber 3b. When the distal ends of other branch optical fibers 6 connected to the O/E converters 10 are connected to these photocouplers 7, the optical fiber 3b and sub-stations 2-1, 2-2, 2-n, . . . are optically connected to each other.
Hence, each sub-station branches information optically transmitted from the main station 1 by the photocoupler 7 and inputs the information to the O/E converter 10. The O/E converter 10 photoelectrically converts the information into an electric signal. The electric signal is demodulated by the sub-station demodulator 11. Control information contained in the demodulated information is supplied to the E/O converter 4 via the sub-station controller 8 to control electrooptical conversion. On the other hand, information transmitted from the sub-station is modulated by the sub-station modulator 9, and the modulated information is converted by the E/O converter 4 into an optical signal. The optical signal is sent onto the optical fiber 3a via the branch optical fiber and photocoupler 7.
Furthermore, in FIG. 2, reference numeral 12 denotes a main station controller; 13, a main station modulator; 14, a main station E/O converter; 15, a main station O/E converter; 16, a main station demodulator; and 17, a beat detector, which construct the main station 1. In the main station 1, an optical signal transmitted via the optical fiber 3a is received by the O/E converter 15, and an electric signal obtained by photoelectric conversion is supplied to the beat detector 17 and main station demodulator 16. The electric signal is demodulated by the main station demodulator 16, and the demodulated signal is output. The beat detector 17 detects beat noise from the electric signal output from the O/E converter 15 after photoelectric conversion.
The main station controller 12 modulates a transmission signal using the main station modulator 13 while controlling the modulator 13 in accordance with the detection output from the beat detector 17. The modulated signal is converted by the E/O converter from an electric signal into an optical signal. The optical signal is then output onto the optical fiber 3b. 
The beat detector 17 provided to the main station 1 detects beat noise. In the main station 1, the beat detector 17 detects beat noise produced when the wavelength spacing between certain sub-stations becomes small, by monitoring the power of beat noise.
Upon detection of the beat noise, in the main station 1, a control means (not shown) supplies a wavelength change command to the sub-stations 2-1, 2-2, 2-n, . . . in turn to temporarily change the wavelengths of the sub-stations 2-1, 2-2, 2-n, . . . in turn, and to specify source sub-stations that have produced beat noise therebetween by comparison with the power of the beat noise. Then, the control means supplies a wavelength control instruction to the specified sub-station to manage its wavelength.
As described above, in the conventional wavelength multiplex transmission system, the following method has been proposed as a measure against beat noise. That is, the beat detector checks whether or not beat noise has been produced by monitoring noise power in the beat noise band in the photoelectrically converted reception signal. If the detector determines that beat noise has been produced, the wavelengths of the sub-stations are changed in turn, and changes in beat noise are compared with those in wavelength to specify the sub-station that has caused the beat noise.
However, this method suffers the following problems since a wavelength change command is supplied to sub-stations in turn to change their wavelengths:
[i] If the number of sub-stations is large, a long time is required for selecting each sub-station since a wavelength change instruction must be repetitively issued to the sub-stations.
[ii] This wavelength change instruction may produce another beat noise between sub-stations that have otherwise kept an appropriate wavelength spacing.
[iii] The wavelength control algorithm is complicated.
Therefore, development of a technique that can specify a sub-station that is a beat noise source from a plurality of sub-stations without influencing the sub-stations at normal wavelength positions is in strong demand.
It is therefore an object of the present invention to provide an optical communication system comprising a beat noise detector which allows a main station alone to specify a sub-station that is a beat noise source without issuing any wavelength change instruction to sub-stations.
According to the present invention, the wavelengths of sub-stations at normal wavelength positions need not be controlled, and the wavelength of only a sub-station that has caused beat noise is controlled to suppress beat noise. Hence, wavelength control between the main station and sub-stations can be quickly done by a simple algorithm.
In order to achieve the object, according to the first aspect of the present invention, there is provided an optical communication system comprising:
a plurality of sub-stations; and
a main station connected to the plurality of sub-stations via an optical transmission path,
wherein the plurality of sub-stations respectively comprise modulation means for modulating a wavelength of an optical signal containing information signals, by using a control signal having a unique frequency allocated to the sub-station, which is transmitted from the respective sub-station to the main station via the optical transmission path, arbitrary pairs of control signals having different frequency differences, and
the main station comprises:
extraction means for extracting a beat noise component from the optical signals modulated by the modulation means; and
determination means for determining two sub-stations that have produced beat noise on the basis of frequency difference of an arbitrary pair of control signals contained in the beat noise component extracted by the extraction means.
According to the second aspect of the present invention, there is provided an optical communication system of the first aspect, wherein the modulation means directly modulates a semiconductor laser diode in intensity by using the information signals and control signal.
According to the third aspect of the present invention, there is provided an optical communication system of the first aspect, wherein the modulation means comprises:
generation means for generating a first optical signal by directly modulating a semiconductor laser diode by using the control signal; and
an external optical modulator for modulating the first optical signal by using the information signal so as to obtain the optical signal modulated by the modulation means.
According to the fourth aspect of the present invention, there is provided an optical communication system of the first aspect, wherein when the number of the plurality of sub-stations is L (L is an integer not less than 3), fm1 to fmL respectively represent frequencies of the control signals of the optical signals of the L sub-stations, and two frequency differences between arbitrary pairs of the control signals are respectively given by:
xcex94fm=|fmMxe2x88x92fmN|, xcex94fmxe2x80x2=|fmOxe2x88x92fmP|
(Mxe2x89xa0N, Oxe2x89xa0P, Mxe2x89xa0O, 1xe2x89xa6M, N, O, Pxe2x89xa6L, and M, N, O, and P are integers)
the two frequency differences satisfy:
xcex94fmxe2x89xa0Qxc3x97xcex94fmxe2x80x2
(Q is an integer) and,
a maximum modulation frequency of information signals S to be transmitted from the sub-stations 1 to L to the main station,
modulation signals fs1 to fsL falling within different frequency bands in the sub-stations 1 to L,
a frequency difference between an arbitrary pair of the modulation signals fs1 to fsL, and
the frequencies fm1 to fmL and the frequency difference xcex94fm satisfy:
maximum modulation frequency of information signal S less than xcex94fm less than fm1 to fmL, xcex94fs, fs1 to fsL
According to the fifth aspect of the present invention, there is provided an optical communication system of the first aspect, wherein the determination means comprises:
detection means for detecting the beat noise component extracted by the extraction means;
filter means for respectively extracting frequency difference components of the control signals from the beat noise component detected by the detection means; and
specifying means for specifying the two sub-stations that have produced the beat noise on the basis of the frequency difference components extracted by the filter means.
According to the sixth aspect of the present invention, there is provided an optical communication system of the first aspect, wherein the determination means comprises:
detection means for detecting the beat noise component extracted by the extraction means;
formation means for sequentially forming signals having frequencies corresponding to the frequency differences of the control signals of the sub-stations;
correlation means for outputting a signal indicating correlation between the beat noise component detected by the detection means, and the signal formed by the formation means; and
specifying means for specifying the two sub-stations that have produced the beat noise on the basis of the signal output from the correlation means.
According to the seventh aspect of the present invention, there is provided an optical communication system comprising:
a plurality of sub-stations; and
a main station connected to the plurality of sub-stations via an optical transmission path,
wherein the plurality of sub-stations respectively comprise
modulation means for directly modulating a semiconductor laser diode in intensity by using information signals, and generating an optical signal which is transmitted from the respective sub-stations to the main station via the optical transmission path, and the main station comprises:
extraction means for extracting a beat noise component from the optical signals modulated by the modulation means;
detection means for detecting the beat noise component extracted by the extraction means;
means for acquiring the information signals of the sub-stations from the optical signals transmitted, and sequentially outputting signals indicating correlation between the beat noise component detected, and the information signals acquired; and
specifying means for specifying the two sub-stations that have produced the beat noise on the basis of the signals indicating the correlation sequentially output.
According to the eighth aspect of the present invention, there is provided an optical communication system of the seventh aspect, wherein the information signals are modulated by using a digital signal, and the means for sequentially outputting executes correlation processing within one symbol time of the digital signal.
According to the ninth aspect of the present invention, there is provided an optical communication system of the seventh aspect, further comprising:
phase shift means for shifting phases of the information signals acquired, and sequentially outputting signals indicating correlation between the beat noise component detected, and the information signals shifted,
wherein the specifying means specifies the two sub-stations that have produced the beat noise based on the signals indicating correlation between the beat noise component detected, and the information signals shifted.
According to the 10th aspect of the present invention, there is provided an optical communication system of the ninth aspect, wherein the phase shift means shifts 90xc2x0 the phases of the information signals acquired.
According to the 11th aspect of the present invention, there is provided an optical communication system of the first aspect, wherein the main station further comprises:
means for outputting a wavelength control signal, for changing the wavelength of the optical signal to be transmitted to one of the two determined sub-stations, and
the sub-station further comprises:
changing means for changing the wavelength the optical signal to be transmitted to the main station on the basis of the wavelength control signal output.
According to the 12th aspect of the present invention, there is provided an optical communication system of the first aspect, further comprising:
increasing means for increasing power of the optical signal transmitted from a new sub-station connected to the transmission path while detecting beat noise by the determination means;
stop means for stopping the increase in power of the optical signal transmitted from the newly connected sub-station when the determination means detects the beat noise caused by the power of the optical signal transmitted from the newly connected sub-station and increased by the increasing means;
changing means for changing a wavelength of the optical signal transmitted from the newly connected sub-station and a wavelength of the optical signal transmitted from another sub-station when the stop means stops the increase in power of the optical signal transmitted from the newly connected sub-station; and
means for increasing the power of the optical signal transmitted from the newly connected sub-station when the changing means has changed the wavelengths of the optical signals transmitted from the newly connected sub-station and the other sub-station.
According to the 13th aspect of the present invention, there is provided a system of the seventh aspect, wherein the main station further comprises:
means for outputting a wavelength control signal, for changing the wavelength of the optical signal to be transmitted to one of the two determined sub-stations, and
the sub-station further comprises:
changing means for changing the wavelength the optical signal to be transmitted to the main station on the basis of the wavelength control signal output.
According to the 14th aspect of the present invention, there is provided a system of the seventh aspect, further comprising:
increasing means for increasing power of the optical signal transmitted from a new sub-station connected to the transmission path while detecting beat noise by the determination means;
stop means for stopping the increase in power of the optical signal transmitted from the newly connected sub-station when the determination means detects the beat noise caused by the power of the optical signal transmitted from the newly connected sub-station and increased by the increasing means;
changing means for changing a wavelength of the optical signal transmitted from the newly connected sub-station and a wavelength of the optical signal transmitted from another sub-station when the stop means stops the increase in power of the optical signal transmitted from the newly connected sub-station; and
means for increasing the power of the optical signal transmitted from the newly connected sub-station when the changing means has changed the wavelengths of the optical signals transmitted from the newly connected sub-station and the other sub-station.
According to the 15th aspect of the present invention, there is provided a control method for an optical communication system comprising the steps of:
transmitting optical signals containing information signals from a plurality of sub-stations to a main station and control signals unique to the sub-stations via a transmission path, arbitrary pairs of the control signals having different frequency differences;
extracting a beat noise component from the transmitted optical signals; and
determining two sub-stations that have produced beat noise on the basis of frequency difference of an arbitrary pair of control signals contained in the extracted beat noise component.
According to the 16th aspect of the present invention, there is provided a control method of the 15th aspect, wherein when the number of the plurality of sub-stations is L (L is an integer not less than 3), fm1 to fmL respectively represent frequencies of the control signals of the optical signals of the L sub-stations, and two frequency differences between arbitrary pairs of the control signals are respectively given by:
xcex94fm=|fmMxe2x88x92fmN|, xcex94fmxe2x80x2=|fmOxe2x88x92fmP|
(Mxe2x89xa0N, Oxe2x89xa0P, Mxe2x89xa0O, 1xe2x89xa6M, N, O, Pxe2x89xa6L, and M, N, O, and P are integers)
the two frequency differences satisfy:
xcex94fmxe2x89xa0Qxc3x97xcex94fmxe2x80x2
(Q is an integer) and,
a maximum modulation frequency of information signals S to be transmitted from the sub-stations 1 to L to the main station,
modulation signals fs1 to fsL falling within different frequency bands in the sub-stations 1 to L,
a frequency difference between an arbitrary pair of the modulation signals fs1 to fsL, and
the frequencies fm1 to fmL and the frequency difference xcex94fm satisfy:
maximum modulation frequency of information signal S less than xcex94fm less than fm1 to fmL, xcex94fs, fs1 to fsL
According to the 17th aspect of the present invention, there is provided a control method of the 15th aspect, wherein the step of determining the two sub-stations comprises the steps of:
detecting the extracted beat noise component;
extracting frequency difference components of the control signals from the detected beat noise component by using the filters, respectively; and
specifying the two sub-stations that have produced the beat noise on the basis of the extracted frequency difference components.
According to the 18th aspect of the present invention, there is provided a control method of the 15th aspect, wherein the step of determining the two sub-stations comprises the steps of:
detecting the extracted beat noise component;
sequentially forming signals having frequencies corresponding to the frequency differences of the control signals of the sub-stations;
outputting a signal indicating correlation between the detected beat noise component, and the formed signal; and
specifying the two sub-stations that have produced the beat noise on the basis of the output signal.
According to the 19th aspect of the present invention, there is provided a control method for an optical communication system comprising the steps of:
transmitting optical signals containing information signals from a plurality of sub-stations to a main station via a transmission path;
extracting a beat noise component from the optical signals transmitted;
acquiring information signals of the sub-stations from the optical signals transmitted;
outputting signals indicating correlation between the beat noise component extracted and information signal acquired; and
determining two sub-stations that have produced beat noise based on the signals outputted.
According to the 20th aspect of the present invention, there is provided a control method of the 19th aspect, wherein the step of determining the two sub-stations comprises the steps of:
shifting phases of the information signals acquired; and
outputting second signals indicating correlation between the detected beat noise component, and the phase-shifted signals,
wherein the step of determining determines the two sub-stations that have produced the beat noise based on the second signals.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.